IAVERAGE will average a series of frames (answer the questions to
specify the names of the frames to be averaged).
Display a spectroscopic frame on the monitor. Then type SKYSUB, which
will ask if the display was via IMAGE or SIMAGE. It will allow you to
place the cursor on one side, then the other side of the object
spectrum, then choose pairs of cursor positions to define sky
intervals. Multiple sky intervals are allowed. When you are finished
with this phase type Q. SKYSUB will then calculate the correctly
scaled object minus sky spectrum, where the object spectrum is that
summed across the indicated interval. A disk file is written with the
1d spectrum; its name is typed on the terminal screen. You can then
plot the sky-subtracted spectrum using SPLOT.
ISTAT will calculate statistical properties (means, dispersions, etc.)
of selected areas of images (or of 1d spectra).
Two useful FIGARO words are those which do 1d extractions from 2d
images. Note that all 1d extractions of the integer 2d images are
real one-dimensional arrays, hence no overflow or underflow errors
occur during their manipulation. EXTRACT or YSTRACT sum a contiguous
set of rows or columns within a picture to form a 1d line.
Once you have a suitable spectrum, you may wish to apply a wavelength
calibration. This requires also having a 1d arc spectrum formed by
summing rows or columns from the two-dimensional image of an arc lamp.
Wavelength calibration is described in the FIGARO documentation for
ARC. Wavelength tables for Helium, Neon, and Argon are stored and are
listed in Appendix C. You specify the degree of the polynomial you
wish to fit in wavelength versus pixel number. (For CCDs in grating
spectrographs three is enough.) You then identify a few lines, after
which the program will start guessing identifications and asking if
they are correct. In using ARC, it is useful to have already
displayed the arc (with SPLOT or looked at the 2d IMAGE) and have
written down the pixel numbers of a few lines together with a guess
for their wavelengths. Having created one spectrum with a wavelength
calibration, this can be copied to other 1d spectra using the command
XCOPY. SPLOT will then produce plots of counts versus wavelength.
If you already have an arc spectrum identified, and you have another arc spectrum which is identical, but spatially shifted (from, for example, flexure of the spectrograph during the course of the night, a small change in grating angle, or from a multislit spectroscopic frame), ARC can also handle this suitably. You must do a full fit to one of the arc spectra, and then merely identify one line in each of the others, so that ARC can compute the correct shift, and reidentify the lines. The procedure is described in Section 14.14 of the FIGARO documentation (i.e., the section on ARC in the FIGARO techniques documentation). Pay particular attention to the end of Section 14.14, the subsection called ``working with shifted spectra''.
All the additional FIGARO words for extinction, fluxing, etc., are believed to work on the Palomar Microvaxes -- see the FIGARO documentation for details on how to use them.